Conventional fin strips wound around tubes of, for example, heat exchangers for recovering heat from exhaust gas produced by combustion are those having an L-shaped cross section and fins formed by slitfing the edge thereof and those having a U-shaped cross section and fins formed by slitfing the edge thereof. Disclosed in U.S. Pat. No. 3,652,820 (Patent family: Japanese Utility Model Publication No. 55-42140) is a heat exchanger employing finned tubes each formed by winding a fin strip 4 having fins 3 separated by slits around a tube 1 and twisfing the fins 3 as shown in FIG. 12.
The inventors of the present invention proposed in Japanese Patent Laid-open Publication No. 4-126997 a heat exchanger employing finned tubes each having fins inclined at an inclination angle .theta. (FIG. 3) in the range of 2.degree. to 20.degree. to a straight line perpendicular to the axis of the tube, and having a fin slit ratio H/h, where h is the height of the base 2 of the fin strip 4 from which the fins 3 extend, i.e., a portion of the fin strip 4 not provided with any slits (FIG. 4), and H is the overall height of the fin strip 4 including the fins 3, not smaller than 1.5, preferably, in the range of 3 to 15. In the prior art fin strip having an L-shaped cross section and fins formed by slitfing the peripheral portion thereof, the fin strip is attached to a tube at a setfing angle, i.e., a fin inclination angle .theta., of zero degree, nothing is considered about the fin slit ratio (H/h) mentioned in the invention previously proposed by the inventors of the present invention, and any particulars about the twist angle of the fins of the fin strip are not taken into consideration. The inventors of the present invention had not taken particulars about the twist angle of the fins of the fin strip into consideration in making the invention previously proposed by the inventors.
Accordingly, it is an object of the present invention to provide a finned tube of a construction having an improved heat transfer coefficient. Another object of the present invention is to provide a heat exchanger having an improved heat transfer coefficient. A further object of the present invention is to provide a heat recovery system provided with a heat exchanger having an improved heat transfer coefficient.